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The two images on the left show radar images of this cell at different levels. The top image is the base reflectivity scan, about 1,000 feet off the ground. The lower image is the 4.5 degree scan, which shows this cell about 10,000 feet off the ground. The photographer's position is denoted by the pink arrow (looking east). Notice that the thunderstorm is detected 10,000 feet of the ground, but nothing shows up in the base reflectivity scan. This is because the cell is in its infant stages and consists primarily of an updraft but no downdraft. As the updraft pushes upward, it causes moisture to condense and precipitation forms. At this point, the precipitation is still aloft in the cloud and has not yet reached the ground. This is better seen in the image on the right, in which a derived radar cross section is superimposed onto the photograph. You can clearly see a "core" of precipitation that is completely elevated and is not yet reaching the ground. You can see why meteorologists will often use higher radar tilts to anticipate developing thunderstorms before anything shows up in the lowest radar scan.